The invention resides in a method and an apparatus for cleaning industrial gases by the removal of solid and liquid particles as they or contained for example in the gases generated by municipal waste combustion in the metallurgical, chemical and other industrial plants.
The filtering of gases containing mainly submicron-particles is an urgent practical problem. The effectiveness of presently available gas purification equipment is not satisfactory.
If possible at all, purification of gases including submicron particles requires high gas speeds. Often cyclones are used herefor, wherein the gas flow is rotated utilizing the centrifugal forces for the particle separation. This however consumes a relatively large amount of energy. In electrostatic separators, on the other hand, the number of electric fields or the length of the high voltage electrodes or of the grounded electrodes must be increased. Again, this increases the energy consumption for the electrostatic charging of the particle and also the size of the gas purification plant. In wet separators, the collection of the submicron particles substantially increases the need for the spray liquid and also requires a high relative speed between the water droplets and the gas flow.
For the collection of the submicron particles, different microporous filters are used such as ceramic filters, filter sacks and bags etc. (see U.S. Pat. No. 4,029,482, U.S. Pat. No. 3,999,964).
The effectiveness of most of these devices however is limited by the low velocity of the gas flow. In many devices, the collection of submicron particles also causes a high-pressure loss, which results in high-energy consumption. Also, the filters need to be cleaned frequently by pneumatic pulses or washing.
The collection of submicron particles can be improved by saturating the gas with water vapors. The water vapor condensation on particles, the particle charge in an electric field and their discharge by the gas flow is described for example in U.S. Pat. No. 4,222,748 or FR 2,483,259 or DE 2,235,531 or CA 2,001,990.
The known technical solutions have several disadvantages: For the electrical charging of the particles, long arrangements of electrodes are needed for a corona discharge in the space between the electrodes. These electrode systems require high voltages and generate an electric field with a non-homogeneous distribution in the charging zone. This does not provide for an effective electric charging of the particles in the gas at all locations in the space between the electrodes.
Ionization devices are also used for electrically charging particles. However, this requires several ionization devices, which renders the gas purification plant relatively complex. The high voltage ionization devices require large amounts of compressed air and therefore increase the energy consumption.
Filters or absorbers washed by water require large amounts of water for spraying and increase the pressure losses in the gas purification plant.
It is the object of the present invention to provide a gas purification apparatus wherein gases can be purified with improved efficiency.